Known examples of applications of a secondary battery include: an electric power storage system; and a power supply such as an electric vehicle power supply system for supplying a large amount of electric power. In particular, a lithium ion battery has advantages such as high energy density, high input/output density, and a long cycle life, as compared with other kinds of secondary batteries. From the viewpoint of such advantages, it is anticipated that such a lithium ion battery will be applied to such a power supply for supplying a large amount of electric power.
In such an application as a secondary battery for a power supply for supplying a large amount of electric power, the secondary battery is operated as an assembled battery (which is also referred to as a “battery module” or a “module battery”) comprising multiple cells (electric cell). It is conceivable that, in order to allow such an assembled battery to supply a large amount of electric power, the assembled battery preferably has a configuration in which series cell groups each comprising multiple cells connected in series are connected in parallel. This is because, by adjusting the number of cells connected in series, such an arrangement allows the voltage of such an assembled battery to be optimized in a simple manner. In addition, by adjusting the number of cells (series cell groups) connected in parallel, such an arrangement allows the current capacity of such an assembled battery to be optimized in a simple manner.
With such a parallel arrangement comprising such series cell groups, the currents that flow through the respective series cell groups are not necessarily equal to each other. For example, there can be a difference between the currents that flow through the respective series cell groups due to individual differences in the cell characteristics (including degradation in the cell characteristics). Such individual differences in the cell characteristics cannot be completely removed in the manufacturing process. Furthermore, there can be a difference between currents that flow through the respective series cell groups due to a difference in the ambient temperature between the cells. In addition, in some applications, such a difference in the cell ambient temperature is manifested due to the cell layout and due to environmental conditions around the cells such as exposure to solar radiation. In a case in which such a battery system is configured to have a large scale, it is difficult to perform a temperature control operation so as to provide a uniform ambient temperature over all the cells even if air conditioning is performed so as to control the ambient temperature of a housing that houses such an assembled battery.
In order to solve such a problem, a technique is known, which is employed in an assembled battery having a parallel arrangement of series cell groups in order to control the current that flows through each series cell group with high system reliability and with low costs (see Patent document 1, for example).
Furthermore, when a voltage equalizing operation is sequentially performed for multiple batteries, after a large majority of the batteries have been subjected to the voltage equalizing operation, i.e., before only a small minority of batteries have not been subjected to the voltage equalizing operation, in some cases, a problem occurs in the voltage equalization operation for the remaining batteries. For example, in some cases, there is a voltage difference between the batteries that have been subjected to the voltage equalizing operation and the remaining batteries that have not been subjected to the voltage equalizing operation. In this case, when the remaining batteries are directly connected to the battery system so as to perform the voltage equalizing operation, the voltage difference leads to a current flowing between the batteries. In particular, in a stage in which the number of remaining batteries that have not been subjected to the voltage equalizing operation is small, a current flow is concentrated in this small number of remaining batteries, leading to an undesired excessively large current.
In order to solve such a problem, a battery set control system is known, which provides an appropriate operation for removing the voltage difference when multiple battery units to be controlled are connected in parallel (see Patent document 2, for example).
In a case in which the aforementioned voltage equalizing method is employed for a parallel arrangement in which multiple batteries are connected in parallel, there is a difference in the charge/discharge current between a case in which voltage difference compensation is performed between a pair of batteries and a case in which voltage difference compensation is performed between a number of batteries and a single battery. This is due to the internal resistance of each battery and the like. As described above, in a case in which the same voltage equalizing operation is performed giving no consideration to the number of batteries, such an arrangement leads to a problem. For example, in a case in which a low threshold voltage difference is set for the batteries mutually connected to each other, such an arrangement requires a long time period until all the batteries output a uniform voltage. Conversely, in a case in which a high threshold voltage difference is set for the batteries mutually connected to each other, in some cases, in the last connection step or in a connection step in the vicinity of the last connection step, an undesired excessively large current flows through a particular battery thus connected in such a step.
In order to solve such a problem, a battery set control system is known which is capable of executing an appropriate voltage equalizing operation giving consideration to the number of batteries to be subjected to the voltage equalizing (see Patent document 3, for example).